The present invention relates to xe2x80x98an improved process for the preparation of 2-acetyl-1-pyrroline, the principle aroma component of basmati and other varieties of scented rice and also of processed cereal and grain productsxe2x80x99.
The main uses of the present invention are, i) preparation of 2-acetyl-1-pyrroline by a simple process and ii) making the aroma chemical in grams. The flavor chemical is of great value, in the art of application of flavor to foodstuffs like rice and bakery products.
Scented varieties of rice1-6 will have a characteristic, strong aroma when cooked. xe2x80x98Basmatixe2x80x99 varieties in Southeastern Asia, xe2x80x98Dellaxe2x80x99 in America, xe2x80x98Milagrosaxe2x80x99 in Philippines, xe2x80x98Khao Dawk Mali 105xe2x80x99 in Thailand, xe2x80x98Seratus Malamxe2x80x99 in Indonesia and xe2x80x98Heirixe2x80x99 in Japan are examples of such varieties. All over the world, they are highly valued and used in making rice dishes and other food products.
2-acetyl-1-pyrroline was first identified in 1982 by Buttery et al7-8, who described its odour as that of cooked rice and popcorn-like. They also described a method of its synthesis from 2-acetylpyrrole9. Also, a method for its quantitative analysis was developed by Buttery et al10 and refined later by Tanchotikul et al11. It can be used, among other things, to evaluate the flavor of new rice varieties. Steam distillation cum continuous extraction isolation procedure (Likens-Nickerson method) is used for isolating the rice flavor. 2,4,6-trimethylpyridine (collidine) is added as internal standard to the flask containing rice and water. The volatile basic fraction is separated from other volatiles that are extracted into the organic solvent (diethyl ether) by adding dilute sulfuric acid and stirring. In this way, compounds interfering in the GC analysis are removed. After extraction, the basic compounds are regenerated from the aqueous acid by neutralization with excess sodium bicarbonate and re-extracted with ether. The extract after concentration (xcx9cto 0.05 ml), can be analyzed by GC. The amount of 2-Acetyl Pyrroline (AP) is calculated by the following equation.       2    ⁢          -AP  concentration  (ppb)         =                    area  of  AP  peak                    area  of  TMP  peak              xc3x97    150    xc3x97    3.57    xc3x97    5  
A recovery factor of 28% is recommended, for inclusion as determined by the stability of pure 2-AP, under the conditions employed for extraction. Hence, it is necessary to multiply the results based on the internal standard and then by the factor 100/28=3.57 to get the amount of 2-acetyl-1-pyrroline actually present in the cooked rice. The factor 150 is the number of micrograms of the internal standard added and the factor 5 is needed to convert to ppb. Another method by Tonchitkul et al11., employs the SDE method for extraction and selected ion monitoring (SIM) in GC-MS for detection and quantification of 2-acetyl-1-pyrroline and trimethylpyridine. 2-AP is found to be present in all scented rice varieties in quantities ranging from 300 to 750 ppb.
Mol. formula: C6H9ON;
Mol. Wt. 111;
CAS Registry No:85213-22-5;
Structural Formula 
2-acetyl-1-pyrroline can be characterized by a comparison of its retention time and MS data with those of a reference standard or those reported in literature. The mass spectrum of 2-acetyl-1-pyrroline has major peaks at m/z 111(M+, 5% abundance, 83 (11), 69 (11), 68 (8), 55(2), 52 (0.2) and 43 (100), 42 (24) and 41 (50). It""s Kovats index is 1320 on a Pyrex glass capillary column coated with Carbowax 20M. Its IR spectrum in CCl4 displays absorption maxima at 1695, 1620, 1435, 1370, 1340, 1250, 1080, 1000, 975 and 940 cmxe2x88x921.
The compound is a colorless liquid when freshly prepared and purified. It must be protected from light and air and preserved in sealed vials under vacuum at temperatures below xe2x88x9220xc2x0 C. Even under these conditions, it is reported to turn to red and eventually become very dark on long storage. A conjugated polymer, resulting from the coupling of the carbonyl group of one molecule with the 5-position of another molecule, is believed to be the product formed in the process9. For this reason the compound is more stable and better preserved in dilute solutions, especially aqueous ones, for several months at less than 20xc2x0 C.
It seems reasonable to assume that 2-acetyl-1-pyrroline has a similar origin to the bread aroma compound, 2-acetyl-1,4,5,6-tetrahydropyridine12. It is assumed that the intermediate formed after decarboxylation of proline residue, is hydrolyzed with the formation of 1-pyrroline. 2-acetyl-1-pyrroline results from the acetylation 1-pyrroline by 2-oxopropanal. Many syntheses of 2-acetyl-1-pyrroline are reported in recent literature13-17.
Reference may be made to the synthetic method (Buttery et al. J. Agric. Food Chem. 1983, 31, 823-826) starting from 2-acetyl-1-pyrrole. The main drawback of this procedure is that it employs, for reduction, expensive rhodium on alumina catalyst in stoichiometric quantities, because its activity is lost during the reaction. The subsequent oxidative step is inefficient as it affords low yields (less than 10%) and the product requires sophisticated preparative GC for purification.
Reference may be made to the synthetic method of DeKimpe N G et al. (J. Agric. Food Chem. 1993, 41, 1458-1461), wherein pyrrolidine is converted into tripyrroline, which on hydrocyanation yields 2-cyano-1-pyrroline in two steps and subsequently to 2-acetyl-1-pyrroline by Grignard reaction with methylmagnesium bromide or iodide. The main drawback of this procedure is that it has multiple steps and employs potassium cyanide reagent and generation of hydrogen cyanide, both of which are highly toxic.
Reference may be made to another method (Guttmann S, Helv. Chim Acta 1961, 44, 721-744; Hausler and Schmidt, Liebigs Ann. Chem. 1979, 1881-1889; Poisel H and Schmidt U, Chem Ber, 1975, 108, 2547-2553) wherein proline is converted to its methyl ester by passing dry HCl through a methanolic solution of proline or by reaction with thionyl chloride in dry methanol. The drawback of both these methods is that they involve laborious steps of repeated methanol addition and its removal to obtain the hydrochloride of the methyl ester. The methyl ester is then liberated and dried for N-chlorination followed by dehydrohalogenation using triethylamine to afford 2-(methoxycarbonyl)-1-pyrroline. This reaction is very slow ( greater than 24 h) and it also gives rise to side products. This precursor is then treated with methylmagnesium iodide to afford 2-acetyl-1-pyrroline.
Reference may be made to another method (Hoffmann T and Schieberle P, J. Agric. Food Chem. 1998, 46, 616-619) wherein 2-acetyl-1-pyrroline is prepared from N-(tert-butoxycarbonyl)-1-proline in four steps. The drawback of this invention is that it involves sulfur containing 2-pyridylthio intermediate, traces of which could interfere with the olfactory purity of the flavourant.
Reference may be made to yet another study (Favino T F et al. J. Org. Chem. 1996, 61, 8975-8979) wherein 2-acetyl-1-pyrroline is prepared by Penicilin acylase-mediated hydrolysis of 1-[(N-phenylacetyl)amino]4,5-dioxohexane to 1-amino4,5-diketone compound followed by spontaneous ring closure of the latter. The drawback of this method is that, the preparation of N-phenylacetamide derivative involves multiple steps of conversion starting from cis-3-hexenol via the corresponding acetylenic C-6 amine.
The main objective of the present invention is to develop xe2x80x98an improved process for preparation of 2-acetyl-1-pyrroline, a principal basmati aroma componentxe2x80x99, which obviates the aforesaid drawbacks of the processes described in the prior art.
Accordingly, the present invention provides, an improved process for preparation of 2-acetyl-1-pyrroline, the basmati rice flavourantxe2x80x99, by synthesis of 2-acetyl-1-pyrroline by an improved and novel method from proline.
In accordance to the objectives, the present invention provides a process for the preparation of 2-acetyl-1-proline, the said process comprising steps of:
a) dissolving L-pyrroline in dry alcohol, cooling to a temperature in the range of xe2x88x925xc2x0 C. to xe2x88x9215xc2x0 C.,
b) adding thionylchloride to step (a) solution, maintaining the temperature of reactant mixture below 0xc2x0 C.,
c) stirring the reactant mixture of step (b) at a temperature range of 20xc2x0 to 30xc2x0 C. for a time period of 1 h to 4 h,
d) removing the solvent from reactant mixture of step (c) to obtain a residue, which is stored in ice-chest for a time period of 10 hours to 16 hours to obtain a solid mass,
e) adding the solid mass of step (d) to 50% aqueous alkali carbonate solution, extracting with an organic solvent, separating the organic and aqueous layer,
f) drying the organic layer of step (e) over anhydrous sodium sulphate, filtering, evaporating the solvent from filtrate to obtain a residue,
g) distilling the residue of step (f) to obtain 2-carbomethoxy-1-pyrrolidine,
h) dissolving 2-carbomethoxy-1-pyrrolidine of step (g) in ether, cooling to a temperature range of 0xc2x0 to 5xc2x0 C., adding drop wise t-butylhypochlorite over a period of 10 minutes to 30 minutes, stirring at a temperature range of 20xc2x0 to 30xc2x0 C. for a time period of 4 h to 6 h,
i) filtering the solution of step (h), removing the solvent from the filtrate to obtain 2-carbomethoxy-1-pyrroline,
j) preparing Grignard Reagent in dry ether by adding magnesium turning, iodine followed by dropwise methyl iodide and stirring at a temperature in the range of 25xc2x0 C. to 35xc2x0 C. for completion of reaction, cooling the mixture to 0xc2x0 C.,
k) adding an ethereal solution of 2-carbomethyoxy-1-pyrroline to the cooled mixture of step (j), stirring at a room temperature till the reaction is complete,
i) adding 5% aqueous hydrochloric acid to step (k) mixture, extracting with diethyl ether, separating the organic layer, drying the organic layer over anhydrous sodium sulphate, filtering to obtain a filtrate, and
j) distilling the filtrate of step (i) to obtain pure 2-acetyl-1-pyrroline.
In an embodiment of the invention relates to use of dry alcohol, which is selected from a group consisting of dry methanol, ethanol and propanol, preferably methanol.
Another embodiment, the organic solvent used is selected from a group consisting of dimethyl ether, diethyl ether and di-isopropyl ether, preferably diethyl ether.
Another embodiment, the aqueous alkali carbonate solution used, is selected from a group consisting of 50% aqueous sodium carbonate or 50% aqueous potassium carbonate.
Still another embodiment, the dehydrogenation can be performed using potassium acetate.
Still another embodiment, the 2-acetyl-1-pyrroline obtained has a basmati rice flavor or popcorn flavor.
Still another embodiment, the 2-acetyl-1-pyrroline obtained is used in flavoring agent in food products, bakery products, food drinks and rice products.
In an embodiment of the present invention, proline was converted to 2-(carbomethoxy)pyrrolidine by reaction with thionyl chloride under mild conditions.
In yet another embodiment of the invention, 2-(carbomethoxy)-N-chloropyrrolidine was dehydrohalogenated by reaction with potassium t-butoxide or potassium acetate at ambient temperatures.
In another embodiment of the present invention, 2-(carbomethoxy)-1-pyrroline was converted to 2-acetyl-1-pyrroline by a known Grignard reaction with methylmagnesium iodide.